U.S. Pat. No. 4,646,070 discloses an arrangement for determining the aging condition of lubricating oil in internal combustion engines, the dielectric constant of the oil is determined by means of a capacitive sensor, whose electrode measuring chamber is filled entirely with the lubricating oil, and a statement regarding the aging condition of the oil is derived therefrom.
U.S. Pat. No. 4,629,334 discloses a sensor arrangement wherein, by measuring the resistance of an oil disposed between the electrodes of an electric sensor, the aging condition of the oil is inferred. By way of simultaneous measurement of the temperature with an integrated temperature sensor, it is possible to compensate for temperature dependencies of the oil resistance. The electrical evaluation device additionally provides a detector which triggers an alarm signal if conductive oil between the electrodes of the sensor is missing.
Electrical sensors are also used for the level measurement of liquids in containers, for example, U.S. Pat. No. 4,745,893 teaches that the capacitance of a partially liquid-filled capacitor can be evaluated as a level measure.
Further examples of level sensors are disclosed in U.S. Pat. No. 4,924,702 and EP-A 0 378 304. U.S. Pat. No. 4,924,702 describes a level sensor having a measuring capacitor and a reference capacitor. The ratio of the measured values of the measuring capacitor and the reference capacitor is used for determining the level of a liquid in the measuring capacitor. The reference capacitor serves to obtain a level measurement independently of the conductance and the dielectric constant. Further, EP-A 0 378 304 proposes a sensor for the level measurement having a measuring capacitance and a reference capacitance. The level signal is composed of the measuring capacitance and the reference capacitance and thus permits a more precise level determination.